The commercial aerospace industry has been moving towards aircraft designs that have fewer but larger individual fiber composite parts. Conventional manufacturing processes have scaled up accordingly, and have already begun to require tools (for defining the shape of the part) and ovens (for polymer matrix curing) that are large enough to push the size of the buildings that must contain them.
Commercially available cure sensing technologies (such as dielectric cure monitoring) are very expensive, and many rely on the ability to embed non-structural components within the parts, which adversely affects the structural characteristics of the part. Both of these attributes detract from the ability to apply the sensor technologies in a distributed manner.
Current composite manufacturing techniques can be quite unwieldy, and not particularly energy efficient. For advanced high-temperature epoxies used in airplanes, the methods for curing and testing wings are scaled up versions of methods used on much smaller parts. For example, it is now common practice to cure composite jet wings in airplane-sized autoclaves.
High-performance composites are often made with two-part epoxy resins. The curing process of epoxy resins is an exothermic reaction that can be regulated by varying the surrounding temperature and the ratios of the hardener and resin. Current practices for monitoring epoxy curing are either imprecise or very expensive.
A conventional practice for testing whether the epoxy has finished curing is to check ajar of epoxy curing outside the mold. The assumption is, if the jar of epoxy is cured, the part curing in the mold must be cured as well. This is not always accurate, and some more sophisticated methods are also in use. These include monitoring the optical qualities of the resin, checking for a change in opacity in the cured part, or monitoring the dielectric constant of the curing part, waiting for the constant to cross some threshold. These require specialized sensors specifically for the epoxy resins being used, and need to be tuned for each curing process.
Some attempts have been made to use differential scanning calorimetry to check for the end of the exothermic epoxy curing reaction. These attempts remain limited however, for they employ an autoclave to heat the mold and component—not an easily scalable process.